The present invention relates generally to a magnetic bubble memory device provided with ion-implanted propagation tracks. More particularly, the invention is concerned with a magnetic bubble memory device having an ion-implanted strain layer of a uniform strain distribution and a method of manufacturing the magnetic bubble memory device.
The ion-implanted magnetic bubble memory device is characteristically provided with bubble propagation tracks formed by ion-implantation in a magnetic bubble garnet film destined to hold magnetic bubbles, as is disclosed in U.S. Pat. No. 3,828,329. The magnetic bubble memory device having the ion-implanted propagation tracks is suited to be realized with a high density because the propagation track has no gap. More specifically, referring to FIGS. 1a and 1b of the accompanying drawings, the aforementioned propagation track can be realized by forming a mask layer 6 of a photoresist or metal film on a surface of a magnetic garnet film 4 for holding magnetic bubbles 2 and then forming an in-plane magnetization layer 8 by implantation of ions such as hydrogen (H) ions, neon (Ne) ions or the like under the effect of magnetostriction strain brought about in the magnetic garnet film by the implantation of ions. The bubble propagation track extends along the edge 10 of the region on which the mask layer 6 is formed and which has thus undergone no ion-implantation. Upon application of a magnetic field in the in-plane direction, a magnetically charged wall makes appearance along the edge 10, resulting in that the magnetic bubbles 2 are propagated under attraction exerted by the magnetically charged wall.
For driving the magnetic bubbles, the ion-implanted strain layer 8 is firstly required to exhibit above all an implantation-induced anisotropy field of great magnitude in the in-plane direction. To this end, the strain layer has heretofore been formed by implantation of hydrogen ions which can assure that the effective anisotropy field change is substantially in proportion to magnitude of the strain. With the phrase "effective anisotropy field change", it is intended to mean a change in the anisotropy field of the bubble film which is induced due to the ion implantation and constitutes a source of bubble driving force, wherein the change can be expressed by .DELTA.(H.sub.k -4.pi.M.sub.s) where H.sub.k represents magnitude of the anisotropy field and M.sub.s represents density of saturation magnetization. However, it is observed that the effective anisotropy field change of the strain layer formed by the implantation of hydrogen ions is caused to decrease significantly by heat treatment required for stabilization of the characteristics of the bubble device, as will be seen in FIG. 2. Under the circumstance, formation of the strain layer by the implantation of hydrogen ions has heretofore been carried out by implanting hydrogen ions in excess in anticipation for the decrease in the effective anisotropy field change by the heat treatment. However, this method is disadvantageous in that an increased number of uncertain or unstable factors are involved in the manufacturing of the bubble devices which are thus unsuited for the manufacturing on the mass-production basis.
In order that the ion-implanted strain layer serves effectively as the magnetic bubble driving layer, it is necessary that distribution of the strain is flat and uniform. In this connection, it is noted that the strain distribution resulting from the single ion-implantation is similar to the Gaussian distribution 12, as shown in FIG. 3. Accordingly, for flattening the strain distribution, heat treatment is subsequently conducted, whereby the strain distribution brought about, for example, by the implantation of hydrogen ions, as described above, is transformed to a smoothed distribution profile 14 by mitigating the most remarkable strain. Such being the circumstance, multiple ion-implantations such as illustrated in FIG. 4 have heretofore been adopted in an effort to realize the uniform strain layer. According to this multiple ion-implantation method, the implantation of hydrogen ions which assures the effective anisotropy field change of great magnitude is combined with the strain distribution realized by implantation, for example, of Ne-ions, whereby a uniform strain distribution is obtained, as indicated by a curve 16. Although this method can assure uniform strain distribution, it is still disadvantageous in that distribution of the magnetic characteristic of the strain layer is non-uniform and that generation of different types or species of ions as well as a number of ion implanting processes is required because combination of the different species of ions is prerequisite.